This invention relates to cardiac pacers, and relates more particularly to an improved system in which both the sensing and pacing functions are enhanced.
It has been recognized that the energy requirements for cardiac pacing can be decreased by reducing the contact surface area of the pacing electrode. The improved performance of the small contact area pacing electrode results from the generation of a relatively high current density at the electrode-tissue interface for a given energy input. Various commercially available electrodes having relatively small contact areas have been developed to take advantage of this phenomenon. However, it has been found that by decreasing the contact area of the pacing electrode, its ability to sense electrical activity of the heart is reduced. This sensing of cardiac electrical activity is a necessary function in all programmed pacers to prevent competitive pacing and the possibility of cardiac decompensation and ventricular fibrillation.
Heretofore, the sensing problems associated with small contact area electrodes have been compensated for in two ways. One prior art approach has been to increase the sensitivity of the associated pacer sensing circuitry to compensate for the reduced sensitivity of the smaller electrode. A second approach has been to simply increase electrode size. However, if the sensitivity of the pacer electronics is increased, sensitivity to contaminating electrical fields and other interference sources is correspondingly increased. Sensitivity to external interference increases the risk of arrhythmia and may cause serious medical complications. On the other hand, simply increasing electrode size improves the sensing ability of the electrode, but the advantages inherent in a small contact area electrode, namely high current density and decreased energy requirements, are compromised.
While it has thus been recognized that a small contact area electrode is desirable for optimal pacing, a workable system for overcoming the disadvantages associated with using such an electrode for sensing has not heretofore been developed. Accordingly, there is a well-defined need for an electrode system in which both the sensing and pacing functions are enhanced.
Representative prior art electrode systems having electrode contact areas of different sizes are shown in U.S. Pat. Nos. 3,253,595 and 3,815,611. Neither of these references discloses a system in which both sensing and pacing functions are enhanced, however. The general concept of utilizing an electrode in conjunction with an electrical decoupling network is shown in U.S. Pat. No. 3,625,201. None of these references shows or suggests the novel electrode system disclosed herein. Similarly, currently available small contact area electrode systems, which utilize the smaller electrode for both pacing and sensing, do not enhance both the sensing and pacing functions.